Circuit-breaking apparatus



Nov. 10, 1964 H; D. EPSTEIN 3,156,849

CIRCUIT-BREAKING APPARATUS Filed March 16, 1959 FIGI. M

LOAD

United States Patent '0 F 3,156,849 CfitCUlT-BREriKiNG ATPARATUS Henry David Epstein, ainhridae, Mass, assignor to Texas instruments incorporated, Bailas, Tern, a corporation of Delaware Filed Mar. 16, 195%, der. No. 799,517 Claims. (Ql. 317-27) This invention relates to circuit-breaking apparatus, and more particularly to suchapparatus which protects against overload of electrical circuits.

Among the several objects of this invention may be noted the provision of circuit-breaking apparatus which can be used to energize and deenergize any conventional electrical load and additionally provide overload protection with time delay; the provision of such apparatus in which large overload currents can be handled without damaging components of the apparatus; the provision of apparatus of the class described which optionally may be associated in heat-exchange relationship with an electrical load, such as an e] ctric motor, etc; and the provision of circuit-breaking apparatus which will substantially instantaneously open an electrical circuit during heavy overload conditions and which may conveniently be made trip tree. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the constructions hereinafter described, the scope of the invention being indicated in the following claims.

In the accompanying drawings, in which one of various possible embodiments of the invention is illustrated,

FIG. 1 is a schematic circuit diagram of one embodiment of circuit-breaking apparatus of the present invention; and,

FIG. 2 is an exemplary physical embodiment of the apparatus schematically depicted in FIG. 1.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Referring now to the drawings, the circuit-breaking apparatus of the present invention includes a differential comprises a main relay winding or coil MW and a bucking relay winding or coil BW wound on a magnetic relay core RC, which may be or" any of the customary ferromagnetic materials such as silicon steel (laminated if this appartus is to be incorporated in an AC. circuit), ferrite, iron, etc. The magnetic circuit constituted by core-RC is completed by a relay armature RA pivoted atone end. The two relay coils MW and BW are serially connected with switchISW in an electrical circuit between any electrical load, such as a motor, etc., and a source of electrical power, either A.C. or D.C., as indicated at L1 and L2. Thermistor TH, a resistance element with a negative temperature coeflicient of resistivity, is shunt-connected across buckingwinding BW.

Switch SW includes a first switch arm SAl pivoted at one end and carrying a first contact Cl at its other end, and a second similar switch arm 5A2, also pivoted at one extremity and having a second contact C2 afixed to its oppositeend. A manually operable switch handle H, rotatable between an OFF or switch-open position (as indicated in dashed lines) and an ON or switc -closed position (solid lines), includes a lobe HL adapted to engage switch arm SAl and move it downwardly so that contact C1 abuts contact C2. The handle H is biased toward its OFF or counterclockwise position by the action of a tension spring SP1, one end of which is connected to a projection HP of handle H. Each of switch arms SAl and 5A2 is indented so as to form bosses Bi and B2, respectively. The depressed portion on the reverse side of boss Bl constitutes a detent engageable by the lobe HL. A compression spring SP2, insulated from Patented Nov. 10, 1964 electrical contact from the conductive metal switch arms by insulating pads P1 and P2, is mounted between switch arms SAT and SAZ to bias them apart. The upward movement or counterclockwise rotation of 8A2 is limited by an insulated stop ST affixed to the frame of the relay. in order to bias relay arms SAT and 8A2 upwardly against handle H and to maintain relay armature RA in a normally open position, another compression spring SP3 is employed. It will be noted that armature RA is mechanically linked or associated, as diagrammatically shown at Ll(, with switch arm 5A2, and also arm SAT via spring SP2.

Operation is as follows:

Upon an electrical power source being connected across Ll, L2 and manual operation of handle H to its switchclosed or ON position, current will flow in the series electrical circuit constituted by switch SW, the parallelconnected bucking winding BW and thermistor TH, main winding coil MW and the electrical loa Thermistor TH, which constitutes an electrical shunt, carries such proportion of the load current as determined by the relative resistances of BW and TH. As the resistance of TH initially, or when relatively cool, is high relative to the resistance or" BW, a substantial portion of the load current is conducted through BW, thus establishing a magnetic flux in the magnetic circuit RC and RA opposing the magnetic flux produced by the full load current flowing through coil MW. It wi l be noted that the coils MW and BW are oppositely wound to produce opposing magnetic flux in the magnetic circuit, or their relative polarities are such as to provide opposing or bucking magnetic fields. The number of turns, size of winding, conductors and parameters of thermistor TH, are conveniently selectable by one skilled in the art to match the characteristics of the electrical load. Thus, for currents normally required to supply the electrical load, the net magnetic flux thereby produced by coils BW and MW is below a first predetermined value necessary to actuate or pull in armature RA against the bias of spring SP3. Upon the current exceeding a predetermined level (e.g., of the normal load current), the temperature of thermistor TH would increase due to increased current flow through it, lowering its resistance and causing an increased proportion of the load current to be shunted through the thermistor, thus decreasing the opposing flux of BW to such an extent that the net magnetic exceeds the aforesaid predetermined level and armature RA will be actuated. Thus, when a fault occurs in the load or associated circuitry so that a load current is drawn in excess of a predetermined level, the armature RA is pulled in. it will be noted that there will be a time delay between the occurrence of the fault and the moment armature RA is actuated. This time differential is dependent on the circuit component parameters selected, the thermal characteristics of the thermistor, the environment of the thermistor (i.e., the ambient temperature of its location, e.g., whether or not it is in heat exchange relation with the electrical load), and the percentage of overload beyond the predetermined amount. if, for example, the electrical load is a motor and a locked rotor condition occurs, the high overload current through BW and MW will cause such a high net or differential magnetic flux that armature RA will pull in instantaneousl is to be noted that these same parameters also determine the actual current at which the relay will be actuated for dcenergization of the circuit. In particular, the temperature of the location of the thermistor and the current through the load determine to a large extent, for a given system, the minimum current level for circuit clearance (ultimate trip current).

As switch arm SA2 is responsive to actuation of armature RA, it too will be rotated around its pivot in a clockwise direction so as to move contact C2 downwardly is actuated so as to break the circuit between the elecw trical power source and the load. This exemplary novel circuit-breaking apparatus is trip free, i.e., the circuit cannot be held closed on an existing overload or fault.

Therefore, if handle H is held in the switch-closed position without removing the cause of the overload contact C2 will be moved away (as shown by the dashed-line position of 5A2) from contact C1 (solid-line position), thereby breaking the circuit. The apparatus of this embodiment of the invention thus can be manually reset after overload tripping, but is trip free.

It will be understood that the thermistor TH or temperature-sensitive resistor device may be mounted in heatexchange relationship with the load, so that temperature rise of the load as well as an increase in the load current will raise the temperature and decrease the resistance of,

thermistor TH. However, thermistor TH may conveniently be thermally isolated from the load and thus selfheated, i.e., responsive only to the magnitude of load current. It will be noted that the number of turns in the bucking coil are preferably somewhat fewer than. those in the main coil MW. The turns ratio between these coils and the size and resistance of the thermistor TH are simply a function of the load and overload currents which will be encountered, and are minor design features well within the knowledge of those skilled in this art. The shunt arrangement of the thermistor TH and coil BW is advantageous, as it permits the use of relatively large coil currents without overloading the normally relatively higher resistance path through the thermistor. It will also be noted that the novel use of a thermistor shunt-connected across the bucking winding of a differential relay provides a very sensitive and reliable means for sensing overload conditions. Not only is the thermis tor not subjected to the entire overload current, but the system or apparatus of my invention is more responsive because of the separation of the sensing function from the switching functions of the apparatus.

in view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a lirnitng sense.

I claim:

1. Circuit-breaking apparatus comprising a relay having a relay armature associated in a magnetic circuit with a first relay coil and a second relay coil, a switch including first and second switch contacts, said contacts and coils being serially connected in an electrical circuit between an electrical load and a source of electrical power, said second coil being adapted to produce magnetic flux in said magnetic circuit opposing magnetic flux produced by said first coil thereby producing a net magnetic flux in said magnetic circuit, said armature adapted to be actuated when said net magnetic flux reaches a predetermined value, trip-free means for closing said switch contacts, means responsive to actuation of said relay armature to open said contacts, and a temperature-sensitive resistor device adapted to change its resistance in response to predetermined temperature change shunt-connected across said second coil whereby upon the current drawn by said load exceeding a predetermined level the resistor resistance changes, thereby changing said net magnetic flux to said predetermined value, and said electrical circuit for said load is broken.

2. In circuit-breaking apparatus as set forth in claim 1, said temperature-sensitive resistor device being positioned in heat-exchange relationship with said load whereby the resistance of said resistor device is responsive both to the magnitude of current drawn by the electrical load and to the temperature of said load.

3. Circuit-breaking apparatus comprising a difierential relay having a relay armature associated in a magnetic circuit with a main relay coil and a bucking relay coil, a thermistor shunt-connected across said bucking coil, said armature adapted to be actuated when the flux in said magnetic circuit exceeds a predetermined value, a switch including a first switch arm carrying a first contact and a second switch arm carrying a second contact, said contacts and coils being serially connected in an electrical circuit between an electrical load and a source of electrical power, a manually operable handle movable between a switch-open position and a switch-closed position in which said first contact is moved against said second contact, means biasing said handle toward said switchopen position, said first switch arm having a detent engageable by said handle, and means biasing said detent into engagement with said operating handle when it is in said closed position to hold it in said closed position, said second switch arm and the last said biasing means being mechanically associated with and responsive to actuation of said armature to open said contacts and release said operating handle, whereby upon the current drawn by said load exceeding a predetermined level the flux in said magnetic circuit exceeds said predetermined value, said electrical circuit is broken and the apparatus is trip free.

4. Circuit-breaking apparatus as set forth in claim 3 in which said handle biasing means comprises a first spring, said detent biasing means comprising a second spring interposed between said first and second switch arms to bias the contacts apart, and which further includes a third spring biasing said relay armature against actuation.

5. Circuit-breaking apparatus as set forth in claim 4 which further includes a stop for said second switch arm limiting the movement thereof in a direction toward said first switch arm.

References Cited in the file of this patent UNITED STATES PATENTS 1,728,551 Jennings Sept. 17, 1929 2,202,535 Pedatella May 28, 1940 2,448,470 Rypinski Aug. 31, 1948 2,801,317 Goldmuntz et a1 July 30, 1957 FOREIGN PATENTS 382,565 Great Britain Oct. 27, 1932 448,614 Great Britain June 11, 1936 557,707 Great Britain Dec. 1, 1943 728,862 Great Britain Apr. 27, 1955 735,755 Great Britain Aug. 31, 1955 

1. CIRCUIT-BREAKING APPARATUS COMPRISING A RELAY HAVING A RELAY ARMATURE ASSOCIATED IN A MAGNETIC CIRCUIT WITH A FIRST RELAY COIL AND A SECOND RELAY COIL, SWITCH INCLUDING FIRST AND SECOND SWITCH CONTACTS, SAID CONTACTS AND COILS BEING SERIALLY CONNECTED IN AN ELECTRICAL CIRCUIT BETWEEN AN ELECTRICAL LOAD AND A SOURCE OF ELECTRICAL POWER, SAID SECOND COIL BEING ADAPTED TO PRODUCE MAGNETIC FLUX IN SAID MAGNETIC CIRCUIT OPPOSING MAGNETIC FLUX PRODUCED BY SAID FIRST COIL THEREBY PRODUCING A NET MAGNETIC FLUX IN SAID MAGNETIC CIRCUIT, SAID ARMATURE ADAPTED TO BE ACTUATED WHEN SAID NET MAGNETIC FLUX REACHES A PREDETERMINED VALUE, TRIP-FREE MEANS FOR CLOSING SAID SWITCH CONTACTS, MEANS RESPONSIVE TO ACTUATION OF SAID RELAY ARMATURE TO OPEN SAID CONTACTS, AND A TEMPERATURE-SENSITIVE RESISTOR 